Methods and apparatus for processing the backsides of wafers

ABSTRACT

In one aspect, a method of processing the backsides of a batch of semiconductor wafers includes receiving a cassette for wafers in its inverted orientation in which the wafers are horizontally orientated with their front faces facing downwardly, removing the wafers sequentially, or as a batch, and processing the upwardly facing backsides of the wafers. In another aspect, an apparatus for processing wafers from a cassette holding wafers horizontally includes a cassette support where the cassette support is formed to receive an industry standard cassette in its inverted position but not in its upright position.

CROSS REFERENCED TO RELATED APPLICATION

A claim to priority is made to U.S. Provisional Application Ser. No.60/615,964, filed Oct. 6^(th) 2004 and British Patent Application No.0422021.6, filed Oct. 5^(th) 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods and apparatus for processing thebacksides of wafers.

2. Description of Related Art

The semiconductor wafer industry uses standard carriers (“cassettes”)for storage, transportation and automated loading of wafers intoequipment. These standards are set by SEMI (Semiconductor Equipment andMaterials Institute) and defined cassette dimensions for various wafersizes. In all cases these cassettes have a defined bottom, typicallywith an ‘H’ bar shape for accurate location upon a matching platform anda top without an ‘H’ bar and sometimes with a handle. The relevant SEMIstandards include but are not limited to SEMI E1, E1.7 and E1.9. The ‘H’bar is also referred to as ‘bar’ and ‘crossbar’.

It is becoming more common to process, inspect or otherwise access thebackside (bottom) of wafers and this frequently requires wafers to beinverted after unloading from a cassette, or to be inverted and loadedinto a cassette upside down. Such processes might include opticalinspection, sputter deposition chemical vapour deposition or plasmaetching. These processes are most efficiently done with the wafer lyinghorizontal upon a support with the process taking place above the wafer.For backside processing the wafer must therefore be facing front-down.An example written procedure taken from Stamford UniversityNanofabrication facility as at Aug. 29, 2003 is as follows: “FlipTransfer” of wafers:

1. Obtain an empty receiver cassette.

-   -   Make sure it is the appropriate kind and level for cleanliness        for your process.    -   Make sure you are wearing clean vinyl gloves

2. Place the receiver cassette upside down over the donor cassette.

-   -   The protruding pins should mate with the alignment holes on both        sides of the cassettes.

3. Grasp both cassettes in both hands and “flip” the donor cassetteupside down onto the receiver cassette. Make sure you have a firm gripon both cassettes and that they are properly aligned and mated;otherwise, you risk dropping (and thus damaging) all your wafers.

4. Your wafers have now been transferred.

For automated wafer inversion, batch transfer equipment is available tolift all the wafers from a cassette typically lying on its back, andtransfer to another cassette that is the opposite way around.

SUMMARY OF THE INVENTION

From one aspect the invention consists in a method of processing thebacksides of a batch of semiconductor wafers including:

(a) receiving a cassette for wafers in its inverted orientation in whichthe wafers are horizontally orientated with their front faces facingdownwardly.

(b) removing the wafers sequentially, or as a batch; and

(c) processing the upwardly facing backsides of the wafers.

Preferably the wafers are returned to an inverted cassette in their“removed” orientation and the unloaded cassette may then be re-invertedor not depending on the next processing step.

From another aspect the invention consists in apparatus for processingwafers from a cassette holding wafers horizontally including a cassettesupport wherein the cassette support is formed to received an industrystandard cassette in its inverted position but not in its uprightposition.

The reception of a cassette in its correct inverted orientation may bedetermined by optical means.

The support may include means for varying its upper surface profilebetween a profile which will accept an upright cassette and a surfacethat will receive an inverted cassette. This can be simply an adaptorplate to be located on the cassette support or it may be a formationwhich can be moved proud of and beneath the support surface.

The apparatus may include means for rotating a cassette through 180°, inwhich case the profile varying means may operate in synchronism with thecassette rotating means.

From a still further aspect the invention may consist in apparatus forprocessing a wafer from a cassette including cassette support, means fordetermining the orientation of a cassette and cassette handling meansfor placing the cassette on the cassette support in a desiredorientation. The orientation determining means may include an opticalscanner for reading a barcode or other visual identity on the cassette.

Although the invention has been described above it is to be understoodthat it includes any inventive combination of the features set out aboveor in the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be performed in various ways and specific embodimentswill now be described, by way of example, with reference to theaccompanying drawings in which:

FIG. 1 is a schematic view of a metal cassette (prior art);

FIG. 2 is a schematic of a top plate of a cassette elevator foraccepting the bottom of a cassette from 100 mm to 200 mm in size (priorart);

FIG. 3 illustrates an embodiment of the invention;

FIG. 4 is a reproduction of SEMI standard E1 FIG. 1A (prior art); and

Exhibit 1 is a reproduction of pages 3 to 8 of SEMI standard E1.9 (priorart).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As has been mentioned above the semiconductor industry uses standard‘carriers’ better known as cassettes with defined bottoms for locatingcassettes in a vertical orientation on a matching plate for removal ofwafers parallel with the plate. It is a characteristic of these platesthat they will only accept the cassette in one bottom-down orientation.In FIG. 1 a cassette 10 has slots to accept a plurality of wafers and abar 14 which together with the bar ends 13 on the bottom 16 of thecassette forms an ‘H’ shape referred to in the SEMI standards. As willbe seen in FIG. 2 a cassette receiving plate 11 of an elevatorschematically shown, in part, at 12 has a formation shown in particularat 17 in its upper surface 15. In this particular example a bar 19 fitswithin a slot 20 and when a cassette is placed correctly upon the plate11 the bar 19 is displaced and actuates a switch or optical device (notshown) to signal the correct placement of a cassette.

It can be seen from the above description that the cassette can only beplaced bottom side down upon the plate for the elevator system 12 tooperate.

The inventors have appreciated the difficulties, additional waferhandling and/or additional equipment requirements of ‘flip transfer’discussed above and have devised a method and apparatus for overcomingthese as illustrated in FIG. 3.

Here an elevator system shown partially at 121 having top plate 111 withtop surface 151 has a formation 171 to receive to top face 18 of acassette 10. Correct inverted location is signalled e.g. by a bar 191being displaced through a slot in top plate 111. movement of the bar issensed and signalled to the wafer processing system (not shown) of whichthe elevator system 121 is a part.

Additionally or alternatively to the physical parts of the top plate111, an optical system 22 may detect visual indicators such as barcodes, cut out or any other suitable mark upon the cassette to signalthe correct inverted orientation and location of the cassette.

Additionally a gripper 23 may be part of or attached to the waferprocessing system and may be connected to a system incorporating theoptical system 22 such that a cassette may be delivered e.g. roboticallyto the wafer processing system and be inverted before being located uponthe top plate 111.

At exhibit 1 is shown part of the SEMI standard for 300 mm cassettesshowing that, whilst these cassettes are visually quite different thanfor the smaller sizes, they still have a defined ‘bottom’ and ‘top’, andaspect of their design to enable optical sensing of orientation(including inversion) and location.

It is an advantage of this invention that wafers are always in cassettesthe correct way up i.e. backside to bottom of cassette and theprocessing apparatus detects and/or receives an inverted cassette. It istherefore much less likely to mistakenly process a wafer front side whena backside process was intended than when wafers have to be loaded intocassettes the wrong way up, as is current practice when using depositionor etching equipment e.g. for depositing backside metals or etchingthrough a wafer vias.

1. A method of processing the backsides of a batch of semiconductorwafers including: (a) receiving a cassette for wafers in its invertedorientation in which the wafers are horizontally orientated with theirfront faces facing downwardly. (b) removing the wafers sequentially, oras a batch; and (c) processing the upwardly facing backsides of thewafers.
 2. A method as claimed in claim 1 wherein the wafers arereturned to an inverted cassette.
 3. A method as claimed in claim 1wherein an unloaded cassette is re-inverted.
 4. A method as claimed inclaim 1 including inverting the cassette from its normal orientation inwhich the wafers have their front faces facing upwardly.
 5. Apparatusfor processing wafers from a cassette holding wafers horizontallyincluding a cassette support wherein the cassette support is formed toreceived an industry standard cassette in its inverted position but notin its upright position.
 6. Apparatus as claimed in claim 5 wherein thesupport includes means for varying its upper surface profile between aprofile which will accept an upright cassette and a surface that willreceive an inverted cassette.
 7. Apparatus as claimed in claim 5 furtherincluding means for rotating a cassette through 180°.
 8. Apparatus asclaimed in claim 7 wherein the profile varying means operates insynchronism with the cassette rotating means.
 9. Apparatus forprocessing a wafer from a cassette including a cassette support, meansfor determining the orientation of a cassette and cassette handlingmeans for placing the cassette on the cassette support in a desiredorientation.
 10. Apparatus as claimed in claim 9 including a cassettesupport formed to receive an industry standard cassette in its invertedposition, but not in its upright position.